In a plural channel AM station carrier system a plurality of central office transmitting and receiving terminals (also referred to as central office channel terminal units or circuits) are connected by a single transmission line to a corresponding number of subscriber transmitting and receiving terminals (also called subscriber channel terminal units or circuits) which are located remotely from the central office. The central office and subscriber terminal units each have a transmitter equipped to transmit a carrier signal of pre-selected frequency and a receiver tuned to receive a carrier signal of a different pre-selected frequency. The carrier signals transmitted over the transmission line from the central office and subscriber channel terminal units are frequency division multiplexed.
The receivers in the central office and subscriber channel terminal units each comprise some sort of frequency selective means for tuning it to receive just one of the incoming carrier frequencies so that each receiver receives a different pre-selected carrier frequency. In this manner each subscriber channel terminal unit is paired with a different central office channel terminal unit to make up a carrier derived circuit or two-way transmission channel.
Prior to this invention it has been the general custom to use bandpass filters as the means for achieving carrier frequency selectively in the receivers of the central office and subscriber channel terminal units. It has also been the general practice to equip the receivers with envelope detectors for effecting carrier signal detection.
In a typical telephone carrier system receiver the frequency selective bandpass filter (hereinafter called a channel or receive bandpass filter) is usually of the LC type and is tuned to a pre-selected carrier frequency so that it effectively rejects all but the desired carrier signal and its sidebands. The envelope detector is connected in the receiver to detect the received carrier signal, and the frequency components resulting from the detection are applied to a low pass filter which has an upper cutoff frequency of about 3000 Hz for separating the voice frequency detection components from the detected carrier frequency component and the other components having frequencies higher than the upper limit of 3000 Hz. In this way voice frequency signals present at the output of the detector will be passed by the low pass filter for transmission to a subscriber's telephone.
Envelope detectors are advantageous because of their low cost, simplicity and reliability in operation. They do, however, have a major drawback in that they create a potential crosstalk problem. The crosstalk problem can develop because the envelope detector will produce the sum and difference frequency components for all combinations of the applied carrier and sideband frequencies. Since any practical channel bandpass filter design will not completely attenuate the unwanted carrier frequencies and their sidebands, they will be applid in partially attenuated form to the envelope detector. As a result, the side frequencies of each unwanted carrier frequency will be demodulated at the envelope detector as frequencies that are in the 300 Hz to 3000 Hz audio range. These demodulated voice frequency signal components from the disturbing channel are therefore within the passband of the receiver's low pass filter and will unavoidably be passed in unattenuated form by the low pass filter. They consequently represent disturbing crosstalk signals that can be heard at the subscriber's telephone.
In the past it has been the general practice to reduce the power in these crosstalk signals to an acceptable level by achieving a more effective rejection of the unwanted carrier frequencies and their sidebands at the receiver's channel bandpass filer. This is done by increasing the order of or the number of pole pairs in the bandpass filter to establish a faster rolloff.
In a carrier system using 8 kHz spacing between adjacent carrier frequencies for example, a three or four pole channel bandpass filter is normally required to reduce the crosstalk to a level that is acceptable under Rural Electrification Administration (REA) standards. For this reason the receive bandpass filters used in prior carrier system receivers are relatively expensive and represent a significant percentage of the cost of the signal receiving equipment.
In an IEEE technical paper published in 1975 and entitled "FDM Subscriber Carrier: Expansion of Electronics In Telephone Plant Technology", synchronous detection of the desired incoming carrier signal was proposed in lieu of envelope detection for the cost-saving purpose of eliminating the channel bandpass filters in a multi-channel AM station carrier system. This IEEE paper discloses a carrier system receiver in which all of the incoming carrier signals are applied to a synchronous detector without any pre-detection bandpass filtering and in which a phase locked loop is used for generating the desired synchronous carrier signal frequency to drive the synchronous detector.
Unlike an envelope detector, a synchronous detector produces difference frequency components which are only the difference between the zero beat frequency (i.e., the frequency of the desired carrier signal) and all of the other carrier and sideband frequencies that pass into the receiver to the input side of the detector. For a carrier frequency allocation having an 8 kHz spacing between adjacent carriers, the closest side frequency in each adjacent channel will synchronously demodulate as a 5 kHz signal component rather than a 3 kHz signal as is the case with an envelope detector.
Of all the unwanted components of detection at the output of the synchronous detector, 5 kHz will be the lowest frequency from a neighboring channel for the 8 kHz carrier frequency spacing example given above. While this 5 kHz signal component is within a person's hearing range, it is above the 3000 Hz cutoff of the receiver's low pass filter and therefore can be attenuated sufficiently by making the filter's rolloff fast enough.
Thus by using the synchronous detector to cause the unwanted signal components to have frequencies above the 3000 Hz cutoff and by making the low pass filter's rolloff fast enough, the crosstalk problem may be avoided.
While advantageously eliminating the channel bandpass filters, the system described in the above-mentioned IEEE paper is subject to a number of troublesome conditions such as the tendency of the phase locked loop to fall out of lock during operation, the tendency of the phase locked loop to acquire lock with a signal of improper frequency, and the tendency of the phase locked loop to become unstable and break into oscillation under certain conditions. To eliminate these problems additional circuitry is required, thus offsetting to an extent the cost advantage derived from eliminating the bandpass filters.